why do you suppose a white belt in bjj was better at takedowns than a black belt in judo?

Because:

1) He doesn't do judo takedowns. He does single/double legs, pulls guard and jumps straight onto the back of his opponent, all of which are frowned upon in sportive judo, which his judoka opponents are unprepared for. Had he tried to fight conventionally, he'd most likely have gotten his ass handed to him by ippon.